Closed-die forging is a technique which can improve mechanical characteristics by crystal grain refining due to forging and the like and can reduce the number of subsequent machining steps, because a member to be forged which has been heated to a forging temperature is forged into a shape close to a final product. Accordingly, the closed-die forging is a technique useful for manufacturing a structural component which is required to have a high-temperature strength in a form of a near net shape, and is often used in manufacturing of a component formed from a superalloy material, for instance, such as a turbine disk of an airplane. However, when the temperature of the member to be forged is decreased during forging, elongation is locally reduced and a crack occurs on the surface of a base material after forging. This occurrence of the surface crack has been a problem particularly in the forging of the superalloy which is a hard-to-work material.
An isothermal forging method of heating a die during forging and a technique of sequentially heating a member to be forged are proposed as a technique for solving the above described problem (Patent Literature 1). However, the technique in Patent Literature 1 is disadvantageous in its cost and efficiency in the case of relying only on this technique, because of being complicated in the facility and the control.
Then, a covering forging method is proposed (Patent Literature 2) in which a heated member to be forged which is covered with another heat-insulation member is forged together with the heat-insulation member. In addition, in a field of free forging, such a technique is proposed (Patent Literature 3) as to interpose a dummy disk formed from stainless steel as a heat-insulation member between the member to be forged and a lower anvil, because a heat loss particularly from the lower face of the member to be forged is a problem in a closed-die forging method in which the member to be forged always contacts a lower die during forging. These techniques can prevent a temperature decrease in the member to be forged at a low cost with high efficiency. In a column of a conventional technology of Patent Literature 1, such a technology is described as to cover the whole of a base material after having been heated with a heat insulating material like a ceramic fiber or a canning material like a stainless steel material, and to forge the base material remaining covered therewith.